Because of being excellent in heat resistance, insulating properties, solvent resistance, cold resistance, etc., polyimide films have been widely employed as materials for electric/electronic parts under computer- or IC-control.
With the recent tendency toward miniaturized and thinned electric/electronic parts under computer- or IC-control, it has been required to miniaturize and thin wiring boards and IC packaging materials too. In substrates carrying electronic parts mounted thereon, lead-free solders are employed from the viewpoint of environmental protection. In its turn, polyimide films have attracted public attention as substrate materials withstanding elevated temperature for soldering.
However, there is a problem to be solved that polyimide films show only poor adhesion strength to conductor layers formed thereon. To metallize polyimide films, therefore, it has been a practice to mechanically roughen the surface by sandblasting to thereby enlarge the contact area of a conductor layer with a polyimide composition, or to employ an undercoating layer made of Cr, Ni, Ti, Co or V and prior to the formation of a conductor layer on a polyimide film by the sputtering method or the like. In case of using a physical roughening method such as sandblasting, projections due to fillers, etc. contained in the polyimide cannot be completely eliminated, which brings about a defect after the metallizing. In case of using a metallic undercoat layer, there sometimes arise undercutting due to the difference in the etching rates of the metallic undercoat layer and the conductor layer in the etching step of forming an electric circuit on the conductor layer, or short circuit between patterns due to the poor etching properties of the metallic undercoat. In case of using Cr, etc., there arises an additional problem of the disposal of the waste etching liquor.